This invention relates generally to electrical propulsion systems for traction vehicles (such as diesel-electric locomotives) equipped with either direct current or alternating current traction motors, and it relates more particularly to improved means for protecting such a system from serious damage in the event of an overvoltage reflected onto a field winding of a synchronous generator as a result of a shorted diode in a power rectifier circuit coupled to an output of the generator.
In a modern diesel-electric locomotive, a thermal prime mover (typically a 16-cylinder turbocharged diesel engine) is used to drive an electrical transmission comprising a synchronous generator that supplies electric current to a plurality of electric traction motors whose rotors are drivingly coupled through speed-reducing gearing to the respective axle-wheel sets of the locomotive. The generator typically comprises a main 3-phase traction alternator, the rotor of which is mechanically coupled to the output shaft of the engine. When excitation current is supplied to field windings on the rotating rotor, alternating voltages are generated in the 3-phase armature windings on the stator of the alternator. These voltages are rectified and applied to the armature and/or field windings of the d-c traction motors or inverted to a-c and applied to a-c traction motors.
In normal motoring operation, the propulsion system of a diesel-electric locomotive is so controlled as to establish a balanced steady-state condition wherein the engine-driven alternator produces, for each discrete position of a throttle handle, a substantially constant, optimum amount of electrical power for the traction motors. In practice suitable means are provided for overriding normal operation of the propulsion controls and reducing engine load in response to certain abnormal conditions, such as loss of wheel adhesion or a load exceeding the power capability of the engine at whatever engine speed the throttle is commanding. This response, generally referred to as deration, reduces traction power, thereby helping the locomotive recover from such temporary conditions and/or preventing serious damage to the engine.
In addition, the propulsion control system conventionally includes means for limiting or reducing alternator output voltage as necessary to keep the magnitude of this voltage and the magnitude of load current from respectively exceeding predetermined safe maximum levels or limits. Current limit is effective when the locomotive is accelerating from rest. At low locomotive speeds, the traction motor armatures are rotating slowly, so their back EMF is low. A low alternator voltage can now produce maximum motor current which in turn produces the high tractive effort required for acceleration. On the other hand, the alternator voltage magnitude must be held constant and at its maximum level whenever locomotive speed is high. At high speeds the traction motor armatures are rotating rapidly and have a high back EMF, and the alternator voltage must then be high to produce the required load current.
In an electric propulsion system, all of the power components (alternator, rectifier, traction motors, and their interconnecting contactors and cables) need to be well insulated to avoid harmful short circuits between the electrically energized parts of these components and ground. The insulation has to withstand very harsh conditions on a locomotive, including constant vibration, frequent mechanical shocks, infrequent maintenance, occasional electrical overloads, a wide range of ambient temperatures, and an atmosphere that can be very wet and/or dirty. If the insulation of a component were damaged, or if its dielectric strength deteriorates, or if moisture or an accumulation of dirt were to provide a relatively low resistance path through or on the surface of the insulation, then undesirably high leakage current can flow between the component and the locomotive frame which is at ground potential. Such an insulation breakdown can be accompanied by ionization discharges or flashovers. The discharge will start before the voltage level reaches its ultimate breakdown value. The dirtier and wetter the insulation, the lower the discharge starting voltage relative to the actual breakdown value. Without proper detection and timely protection, there is a real danger that an initially harmless electrical discharge will soon grow or propagate to an extent that causes serious or irreparable damage to the insulation system and possibly to the equipment itself.
It is conventional practice to provide ground fault protection for locomotive propulsion systems. Such protective systems typically respond to the detection of ground leakage current by overriding the normal propulsion controls and reducing traction power if and when the magnitude of such current exceeds a permissible limit which depends on the magnitude of motor current. See U.S. Pat. No. 4,608,619 and Canadian Pat. No. 1,266,117. Such systems have not been wholly successful in preventing damaging flashovers on the commutators of the traction motors.
In d-c traction motors, carbon brushes rubbing on commutator bars are utilized to provide current to armature windings of the motor. This current establishes a magnetic field in the armature and corresponding magnetic poles. The magnetic poles created in the armature interact with magnetic poles in field windings of the motor to produce torque in the machine. The magnetic poles in the field windings of the motor are established by means of direct current flowing through these windings. The motor includes a plurality of commutator bars equally spaced around one end of the armature, each of the commutator bars being connected to selected windings of the armature for establishing the magnetic poles. As adjacent commutator bars periodically pass under the carbon brushes, the armature coils connected thereto are momentarily short circuited. Since the coils associated with the short circuited commutator bars are displaced from each other, they will be passing through magnetic flux fields created by the magnetic poles of the field windings which are of different magnitudes. Accordingly, a potential difference will exist between the two commutator bars. In the design of an ideal machine the brushes are located between field poles at a point where flux created by the field poles passes through zero in its reversal between adjacent poles of opposite magnetic polarity. This ideal point shifts with changes in armature current since the total flux is the sum of field flux and armature flux. Typically, a commutating pole or interpole is put between adjacent field poles, each commutating pole having a winding which is serially connected in the armature current path so that the flux generated by the commutating pole is proportional to armature current. This method generally serves to minimize changes in the interpole flux thus allowing the brush to transfer current between commutator bars without an undue amount of electrical arcing.
For motors that are subject to heavy overloads, rapidly changing loads, operation with weak main fields, defective brushes, brush bounce, or rough commutators, there is a possibility that the commutating pole action may be insufficient, and a simple sparking at the brushes may become a major arc. For example, at the instant an armature coil is located at the peak of a badly distorted flux wave, the coil voltage may be high enough to break down the air between the adjacent commutator bars to which the coil is connected and result in flashover, or arcing, between these bars. Arcing between commutator segments may quickly bridge adjacent brush holders or spread to the grounded flash ring that usually surrounds the commutator of a d-c traction motor, thereby short circuiting the output lines of the traction alternator. While such flashovers are relatively rare, if one occurs it will usually happen when the locomotive is traveling at a high speed.
Many different systems are disclosed in the relevant prior art for automatically detecting and recovering from flashover conditions. See for example U.S. Pat. No. 4,112,475--Stitt and Williamson. To minimize or avoid serious damage to the traction motor and associated parts of the propulsion system when a flashover occurs, it is desirable to extinguish the flashover before the current being supplied to the faulted motor has time to attain its maximum available short-circuit magnitude. By very rapidly reducing or interrupting such current as soon as the flashover can be detected, the amount of electrical energy in the faulted motor circuit will be kept low enough to prevent permanent damage to the commutator bars, brush holders, and flash ring. This desired high speed flashover protection cannot be obtained by opening the electrical contactor that connects the faulted motor to the rectified output of the alternator, because the opening action of a conventional contactor is too slow and by the time the contactor tips start to separate the fault current magnitude could be so high as to cause undesirable arcing or welding of such tips. The deration function of the propulsion controls cannot be relied on to reduce the initial surge of current that the traction alternator supplies to the faulted motor, because the relevant time constants of the controls and of the alternator field excitation circuit introduce a finite delay between the occurrence of a flashover and the response of the alternator.
Although a-c traction motors do not present the flashover problem of d-c motors, the power system for a-c motors can exhibit a condition, commonly referred to as "shoot-through", which has the same detrimental characteristics of a flashover. In a typical a-c traction motor system, the power output of the traction alternator is supplied to a rectifier circuit which converts the a-c output of the alternator to d-c. This d-c power is then inverted by a solid state inverter into a frequency controlled a-c power for application to the a-c motor. The speed of the a-c motor is controlled by the frequency of the supplied a-c power. The inverter is conventionally arranged to provide 3-phase a-c power and includes a plurality of controllable rectifiers such as silicon controlled rectifiers (SCR) or gate turnoff (GTO) thyristors. Each phase has at least two such devices connected in series between the relatively positive and relatively negative d-c power buses extending from the rectifier circuit. During motoring operation, one of the devices in a phase is always off while the other device is conducting. If both devices were conducting simultaneously, the devices would form a short circuit across the rectifier output buses. Such a condition is referred to as a shoot-through and can result in currents that are of the same magnitude as those which occur during a flashover.
Various failures can contribute to a shoot-through condition. For example, one device may simply fail to commutate off before another device begins conducting. More commonly, one device initially fails to a short-circuit condition and the second device in series with it is gated into conduction resulting in a short circuit between the d-c power buses. As with the flashover fault, the deration function of the propulsion system cannot respond sufficiently fast to prevent damage to the power system.
U.S. Pat. Nos. 5,168,416 and 5,245,495 describe one form of flashover protection circuit for a d-c electric traction motor using a series connected solid state switching device to disconnect the alternator field winding from its power source upon detection of a high current surge characteristic of a flashover. One disadvantage of this system is that the series switching device, e.g., a GTO, must be sized to carry alternator field current during normal system operation. Further, the series device requires forced air cooling to prevent overheating and its stress level is high due to the continuous current it must carry.
As discussed above, the 3-phase synchronous generator in a locomotive propulsion system develops an output voltage which is a function of its rotor shaft RPM and the d-c voltage and current applied to its field windings. The 3-phase output is converted to d-c power by a 3-phase full bridge rectifier connected to the generator armature windings. This rectifier contains fuses which function as protective devices to protect the alternator from overvoltages caused by failure of a device in the rectifier. The devices are typically solid-state diodes and fail to a "short-circuit" condition. In a d-c electric traction motor system, the d-c power is coupled directly to the traction motors. In an a-c motor system, the d-c power is applied to an inverter and inverted to a controlled frequency power.
Both a-c and d-c locomotives require protection for rectifier short-circuit failures and such protection has normally been provided by fuses. The fuses are often a maintenance problem, since they last only about 3.27 years in the most severe locomotive conditions (e.g., pulling coal up a steep grade; i.e. low speed, maximum power, and at the highest output rectifier currents). When a fuse blows the locomotive has to operate at a reduced horsepower or none at all (depending on if it is a d-c or an a-c locomotive). Accordingly, it is desirable to provide a protection system which does not use fuses.